I had a strange problem with XP, in which my machine would literally lock up on me virtually every day, at least once. I wasn't sure if it was a hardware error or not, but I threw Vista on an extra partition and ran that for around a week to play with it, and see.
Incredibly, I was rock solid under Vista, with no crashes or lock ups. So, I've since switched completely. Weird, I know... usually it's the other way around.